1. Field of the Invention
The present invention relates to a method of generating preamble sequence for a wireless local area network device, and more particularly, to a method of generating preamble sequence for a wireless local area network device complying with IEEE 802.11ac standard.
2. Description of the Prior Art
Wireless local area network (WLAN) technology is one of popular wireless communication technologies, which is developed for military use in the beginning and in recent years, is widely implemented in consumer electronics, e.g. desktop computers, laptop computers, personal digital assistants, etc., to provide the masses with a convenient and high-speed internet communication. IEEE 802.11 is a set of WLAN protocols created by the Institute of Electrical and Electronics Engineers, evolved from the former IEEE 802.11a/b/g standard to the current mainstream IEEE 802.11n standard.
IEEE 802.11a/g/n standard adopt orthogonal frequency division multiplexing (OFDM) method, which has advantages of high spectrum utility efficiency and capability of resisting signal attenuation caused by a multipath propagation; whereas, as to transmitters in WLAN systems, the peak-to-average power ratio (PAPR) of modulated signals may easily be excessively high, and a distortion may occur when the modulated signals are processed in radio frequency (RF) circuits of the transmitters, resulting in a decrease of packet detection probability in a receiver. IEEE 802.11n standard is different from IEEE 802.11a/g standard in adding a multiple-input multiple-output (MIMO) technique and other features that greatly enhance data rate and throughput. In addition, in IEEE 802.11n standard the channel bandwidth is doubled to 40 MHz from 20 MHz.
Please refer to FIG. 1, which is a diagram of an IEEE 802.11n packet structure according to the prior art. An IEEE 802.11n packet consists of a preamble portion in the front of a packet and a payload portion after the preamble portion, carrying data to be transmitted. An IEEE 802.11n preamble is a mixed format preamble and is backward compatible with IEEE 802.11a/g standard devices, and includes legacy Short Training field (L-STF), legacy Long Training field (L-LTF), legacy Signal field (L-SIG), high-throughput Signal field (HT-SIG), high-throughput Short Training field (HT-STF), and high-throughput Long Training fields (HT-LTF). L-STF is used for start-of-packet detection, automatic gain control (AGC), initial frequency offset estimation, and initial time synchronization. L-LTF is used for further fine frequency offset estimation and time synchronization. L-SIG carries the data rate (which modulation and coding scheme is used) and length (amount of data) information. HT-SIG also carries data rate and length information, and is used for packet detection so that the mixed format or the legacy format the transmitted packet uses can be detected. HT-STF is used for automatic gain control. HT-LTF is used for MIMO channel detection.
According to the present IEEE 802.11n standard, the lower 20 MHz portion of the 40 MHz preamble is equal to the legacy, IEEE 802.11a/g 20 MHz preamble, and the upper 20 MHz portion of the 40 MHz preamble is a replica of the lower 20 MHz portion with a phase rotation of 90 degrees. The 90-degree rotation on the upper 20 MHz portion is added in order to reduce PAPR when transmitting packets, and therefore the packet detection probability in a receiver is improved.
For the achievement of a higher quality wireless LAN transmission, the IEEE committee creates a new generation IEEE 802.11ac standard, which is IEEE 802.11 VHT (Very High Throughput) standard. Compared to the channel bandwidth of 40 MHz in IEEE 802.11n standard, the channel bandwidth in IEEE 802.11ac standard is increased to 80 MHz. For backward compatibility to wireless devices applying IEEE 802.11a/g/n standard, IEEE 802.ac standard must be designed to be capable of using the 20 MHz channel and the 40 MHz channel selectively. Therefore, besides the backward compatibility of the channel bandwidth, the possibility of excessively high PAPR is also a remarkable issue in designing the preamble.